Chlorinated organic solvent compounds such as tetrachloroethene, trichloroethene, and vinyl chloride are among the most common groundwater contaminants in the USA due to their prior widespread use for numerous industrial and commercial applications. Given their toxicity, widespread occurrence, and complex behavior, chlorinated solvents clearly pose a significant risk to human health and the environment. This proposed project is designed to address and help resolve issues that are critical to the management and closure of chlorinated-solvent contaminated sites. The overall goal of the proposed research is to enhance the accuracy of risk assessments and improve the effectiveness of remediation strategies for sites contaminated by chlorinated solvents. The overarching principle guiding the proposed research is that improving our mechanistic-based, multiple-scale understanding of the distribution, mass-transfer, and mass-flux behavior of chlorinated solvents in heterogeneous systems will enhance risk assessment, site characterization, and remediation efforts. The project is designed to address issues relevant to chlorinated-solvent contaminated sites across the USA, as well as those of particular relevance to sites in the Southwest. The specific aims to be addressed in this project are: (1) Improve our mechanistic understanding of the interfacial and mass-transfer behavior of immiscible liquids in multiphase systems, (2) Investigate the influence of heterogeneity, source-zone aging, and poorly-accessible NAPL on long-term mass-flux dynamics for aqueous and vapor-phase systems, (3) Determine the mass removal, mass flux, and plume contraction behavior of chlorinated solvents at the field scale, (4) Test the efficacy of an innovative method for in-situ characterization of mass transfer and mass flux, and compare the performance of this and several existing methods, (5) Integrate process information across spatial and temporal scales to improve conceptual and mathematical models. The project is designed to accomplish a systematic study of the mass-transfer behavior of chlorinated-solvent immiscible liquids at multiple scales, and to investigate the impact of system properties on mass flux and plume response. The project involves integrated pore-scale, intermediate-scale, and field-scale investigations, as well as mathematical modeling analysis.